As a method replacing a conventional radiography, a radiation image recording and reproducing method utilizing a stimulable phosphor is described, for instance, in U.S. Pat. No. 4,239,968 and is practically used. In the method, a radiation image storage panel comprising a stimulable phosphor (i.e., stimulable phosphor sheet) is employed, and the method involves the steps of causing the stimulable phosphor of the panel to absorb radiation energy having passed through an object or having radiated from an object; sequentially exciting the stimulable phosphor with an electromagnetic wave such as visible light or infrared rays (hereinafter referred to as "stimulating rays") to release the radiation energy stored in the phosphor as light emission (stimulated emission); photoelectrically detecting the emitted light to obtain electric signals; and reproducing the radiation image of the object as a visible image from the electric signals.
In the radiation image recording and reproducing method, a radiation image is obtainable with a sufficient amount of information by applying a radiation to an object at a considerably smaller dose, as compared with the conventional radiography using a combination of a radiographic film and a radiographic intensifying screen. Further, the method is very advantageous from the viewpoints of conservation of resources and economic efficiency because the radiation image storage panel can be repeatedly used in the method, while the radiographic film is consumed for each radiographic process in the conventional radiography.
The radiation image storage panel employed in the above-described method generally comprises a support and a stimulable phosphor layer provided on one surface of the support. However, if the phosphor layer is self-supporting, the support may be omitted.
As the stimulable phosphor layer, there are known not only a phosphor layer comprising a binder and a stimulable phosphor dispersed therein but also a phosphor layer composed of only an agglomerate of a stimulable phosphor, not containing a binder, which is formed through deposition process or sintering process. The present applicant has already applied for patent with respect to a radiation image storage panel having other type of a stimulable phosphor layer in which voids of a stimulable phosphor agglomerate are impregnated with a polymer material (U.S. patent application Ser. No. 184,010). In any of the above-described phosphor layers, the stimulable phosphor emits light (stimulated emission) when excited with an electromagnetic wave (stimulating rays) such as visible light or infrared rays after having been exposed to a radiation such as X-rays. Accordingly, the radiation having passed through an object or radiated from an object is absorbed by the phosphor layer of the panel in proportion to the applied radiation dose, and a radiation image of the object is produced in the panel in the form of a radiation energy-stored image. The radiation energy-stored image can be released as stimulated emission by sequentially irradiating the panel with stimulating rays. The stimulated emission is then photoelectrically detected to give electric signals, so as to reproduce a visible image from the electric signals.
In the case the phosphor layer is provided on a support, a film (i.e., protective film) is generally provided on the free surface (surface not facing the support) of the phosphor layer to protect the phosphor layer from chemical deterioration or physical shock.
When a radiation image storage panel containing a stimulable phosphor is employed in radiography for the medical diagnosis, it is particularly desired that the sensitivity of the panel to a radiation is made as high as possible to reduce the exposure dose for patient and to facilitate the procedure for converting the stimulated emission to electric signals. Accordingly, it is desired to make the sensitivity of the panel as high as possible.
The sensitivity of the radiation image storage panel is essentially determined by the total amount of stimulated emission given by the stimulable phosphor contained therein. The more amount of a radiation the stimulable phosphor absorbs, the more amount of stimulated emission it emits, assuming that the content of the stimulable phosphor is kept at the same level and that its yield of luminescence is constant.
The known and practically used radiation image storage panel absorbs a radiation to the amount of about 60%, and the rest amount (i.e., 40%) of the radiation having useful information for medical diagnosis is not absorbed but passed through the panel. Therefore, in order to improve the sensitivity of a radiation image storage panel, it is desired that the above-mentioned 40% amount of the radiation should be absorbed by the panel as much as possible, and that the efficiency of utilization of a radiation should be improved.